1. Field of the Invention
The present invention relates to a safety current interrupter device. In particular it relates to a safety current interrupter device for use in a secondary cell.
2. Discussion of Prior Art
A secondary cell or electrochemical cell (both terms being equivalent, we shall use the term secondary cell in this description) includes in a known manner an electrode plate group comprising alternating positive and negative electrodes flanking separators, the entire electrode plate group being impregnated with electrolyte. Each electrode includes a metal current collector carrying an electrochemically active material on at least one of its faces. The electrode is electrically connected to output terminal means providing electrical continuity between the electrode and the external application to which the secondary cell is associated. The electrode plate group is placed in a container tightly closed by a lid.
A secondary cell is generally designed to operate at so-called nominal values, ie in a given temperature, current and voltage range. When a secondary cell is operated outside of its nominal conditions, such as accidental overcharging, a short circuit or an external temperature higher than the maximum operating temperature, this creates a risk of explosion. Indeed, such situations can lead to overheating of the electrolyte and the formation of electrolyte vapor. The accumulation of vapor in the container increases the pressure inside the secondary cell, which can lead to violent rupture of the container and the projection of chemicals which are harmful and corrosive to the environment and to people nearby.
Typically, a safety current interrupter device can be built into the lid of the secondary cell. The safety device is able to quickly interrupt the electrical circuit in the battery in the case of operation outside rated conditions. Thus, the battery stops working. The current interrupter function makes it possible to isolate the secondary cell, irreversibly, from apparatus connected to the secondary cell.
The safety device can also include a safety vent function to prevent gas buildup inside the container of the cell, and allow gas to escape when the internal pressure exceeds a predetermined value. The release of gas through the vent avoids an explosion of the cell.
The security device can be activated by pressure in the secondary cell. However gas production in the case of operation outside nominal conditions can not cause sufficient pressure to activate the safety device. Additives can be added in the electrolyte, such as carbon compounds. These additives are released in gaseous form as a result of increasing temperature, thereby increasing the sensitivity of the safety device. However additives disrupt chemical reactions taking place in normal operation of the secondary cell. As a result, the lifetime and performance of the cell is diminished.
Therefore it may be advantageous to supplement the activation of the safety device in response to pressure by operation responsive to temperature.
French patent application FR-A-2,881,580 discloses a secondary cell comprising a current interrupter actuated by an internal pressure of the secondary cell. The device includes an electrical link member handling the current flow. The link member is placed over a membrane, and is electrically insulated from the membrane. Excessive internal pressure of the secondary cell causes a deformation of the membrane leading to rupture of the link member. Nevertheless, the current interrupter can only be activated under the effect of pressure. The current interrupter is consequently wholly dependent on the electrochemical process generating the pressure. It is not possible to activate the device as a result of an increase in temperature.
JP-A-63072062 discloses a safety vent system that can be actuated by the temperature of the cell. The safety venting system consists of a shape memory material connected to one terminal of the cell at one end, and positioned on a portion sealing off the cell at its other end. In the presence of an excessive rise in temperature the shape memory material deforms and breaks the sealing portion of the cell, allowing the escape of gases formed in the secondary cell. The activation of this system depends on the temperature of the cell. However the system cannot be activated by an increase in pressure. Furthermore the system only comprises a safety venting function, and no current interrupter function.
In the above documents, the devices are actuated by either pressure or temperature. None of the devices is sensitive to both pressure and temperature.
International application WO-A-9845887 discloses a secondary cell including in its cover, a first current interrupter actuated by a rise in temperature and a second device operable by excessive pressure in the secondary cell. The first current interrupter device includes a material deformable as a result of an increase in temperature, and a conductive line. Under the effect of increasing temperature, the deformable material causes a change in position of the conductive line and interrupts the current flow. The second current interrupter device includes a metal membrane ensuring the passage of current, on top of a rigid conductive plate. The membrane can be deformed under the action of an excessive pressure on the membrane. The membrane is then driven out of contact with the rigid conductive plate causing an interruption of current flow. The current interrupter devices in that international application also perform a safety venting function.
The secondary cell described in the above document includes a current interrupter function actuated by a pressure or temperature. However, the current interrupter function is split up into two devices: a pressure-actuated device, and a temperature-actuated device. The safety venting function is actuated by a pressure increase. In addition, in order to be deformable, the membrane must not be too thick, meaning that it cannot carry a high current, which limits the secondary cell to low power applications.
There is consequently a need for a current interrupter function that is actuated by pressure and by temperature, combined into one single device, and which is not limited to low power applications.